Optical fibers can carry light signals for great distances without significant loss. Light losses, however, can be significant at a fiber-to-fiber coupling, where two optical fibers are joined. A variety of optical fiber-to-fiber coupling systems have been developed. One class of optical coupling systems uses optical elements such as lens pairs between the ends of the optical fibers. The optical elements collect the light emitted from the source optical fiber and focus the light on the core of the receiving optical fiber. For a high efficiency at coupling optical power into the receiving optical fiber, the focused light needs to be at the proper angles for transmission along the core of the receiving optical fiber. The intervening optical elements can achieve a high coupling efficiency but increase the cost and alignment difficulties involved in creating a fiber-to-fiber coupling.
Butt coupling provides a simple fiber-to-fiber coupling in which the ends of two optical fibers are adjacent to or in contact with each other. This type of fiber-to-fiber coupling is typically made with a tool that adjoins the ends, aligns the optical fibers, and then attaches a physical coupling device and/or adhesive meant to hold the two optical fibers in the established alignment. Several types of errors or differences in the fiber-to-fiber couplings can cause power loss.
One source of power loss is defects in the end faces of the optical fibers. For optimal coupling efficiency the end faces of each optical fiber must be perpendicular to the length of the fiber and free of defects such as chips or roughness.
Another source of power loss in fiber-to-fiber couplings is misalignment of the optical fibers. FIG. 1A, for example, illustrates an example where the centers of the cores 110A and 110B of optical fibers 100A and 100B are offset from each other. FIG. 1B illustrates optical fibers 100A and 100B having an angular misalignment, and FIG. 1C illustrates optical fibers separated by too large of a gap 120. Such alignment errors are inherent in mechanical alignment techniques and result in signal loss. This signal loss can be reduced or minimized using an active alignment procedure that measures the optical power coupled into the receiving optical fiber and adjusts the position and orientation of the optical fibers to maximize power coupling efficiency.
Intrinsic differences in properties of the two optical fibers being coupled are yet another source of power loss in a fiber-to-fiber coupling. FIG. 1D illustrates optical fibers 100A and 100B that have different numerical apertures so that a portion of the light signal from transmitting fiber 100A is at angles that cannot be coupled into receiving fiber 100B. FIG. 1E illustrates a situation where optical fibers 100A and 100B have cores 110A and 110B with different diameters so that less than the full cross-section of the optical signal from the transmitting fiber 100A can be coupled into the receiving optical fiber. Differences in the cladding diameter (as illustrated in FIG. 1F) or differences in the diffractive index profiles of the optical fibers 100A and 100B (as illustrated in FIG. 1G) can result in similar power loss at the optical coupling. This type of power loss is inherent to the optical fibers' properties and variations and is present even if the optical fibers can be ideally aligned.
In view of the current state of the art, optical coupling structures and methods are sought that can reduce optical power loss at the couplings of optical fibers. Such structures and methods would ideally reduce power losses regardless of whether misalignment or differences in the properties of the optical fibers caused the power losses.